As a switching device of the next generation achieving high withstand voltage, low losses, and high temperature resistance, semiconductor devices using gallium oxide (Ga2O3) with a large band gap attract attention and are expected to be applied to power semiconductor devices, such as an inverter. According to NPL 1, such gallium oxide has a band gap that may be controlled by forming mixed crystal with indium or aluminum singly or in combination. Among all, InAlGaO-based semiconductors represented by InX′AlY′GaZ′O3 (0≤X′≤2, 0≤Y′≤2, 0≤Z′≤2, X′+Y′+Z′=1.5 to 2.5) are extremely attractive materials.
PTL 1 describes a highly crystalline conductive α-Ga2O3 thin film with an added dopant (tetravalent tin). The thin film in PTL 1 is, however, not capable of maintaining sufficient withstand voltage. This film also contains many carbon impurities and has unsatisfactory semiconductor properties including conductivity, so that the film is difficult to be used for a semiconductor device.
PTL 2 describes a Ga2O3-based semiconductor element with a p type α-(Alx″Ga1-x″)2O3 single crystal film formed on an α-Al2O3 substrate. The semiconductor element in PTL 2, however, has many constraints to be applied to a semiconductor element due to α-Al2O3 being an insulator and the crystal quality with some problems. The MBE technique requires ion implantation and heat treatment at high temperatures to obtain a p type semiconductor. The p type α-Al2O3 itself is accordingly difficult to be achieved, and in reality, the semiconductor element in PTL 2 itself is difficult to be achieved.
NPL 2 also describes that an α-Ga2O3 thin film can be formed on sapphire by the MBE technique. The description, however, mentions that the crystal grows up to a film thickness of 100 nm at a temperature of 450° C. or less while the quality of the crystal becomes poor with a film thickness more than that and a film with a film thickness of 1 μm or more cannot be obtained.
There is therefore an expectation of an α-Ga2O3 thin film with a film thickness of 1 μm or more and having undegraded crystal quality.
PTL 3 describes a method of manufacturing a crystalline oxide thin film by mist CVD using bromide or iodide of gallium or indium.
PTLs 4 to 6 describe a multilayer structure having a semiconductor layer with a corundum crystal structure and an insulating film with a corundum crystal structure, both laminated on a base substrate with a corundum crystal structure.
PTLs 3 to 6 are publications on patents or patent applications by the present applicant, and at the time of each application, a crystalline thin film having a film thickness of 1 μm or more was not yet able to be obtained. In addition, all films obtained in the methods of PTLs 3 to 6 were not able to be peeled off from the substrate in reality.